It depends upon your perspective, but in the larger scheme of things I would say not. Vampires would be either a pre-existing phenomenon (and we just found out) or a new introduction, but either way they would be a biological phenomenon and thus accommodated into the ecologies of Earth. It might not be so great from the human perspective, but ecologically it would not be a problem because all ecologies absorb changes and return to a relative state of equilibrium.

They're too lazy to be a serious problem :)

If all the folklore was assumed correctly about vampires, humans would adapt in never going outside after dark, carry crosses and garlic at all times, never allow total strangers into their homes, and install ultraviolet lights in everything.

What kind of vampires are we talking about here? The problem of speculating mythologies as if real is there have probably been so many twists and spin-offs on it, I'm sure there is more than ONE kind of vampire. Unless redefined, I will merely speculate on the simplest and least presumptuous idea of a vampire with just enough traits I'd deem worthy enough to call a genuine vampire. Ability to turn into a bat, immortal, fangs, and feeds off human blood possibly other kinds. I don't see why such beings couldn't just take our blood with a syringe thus not harming us, thus we can coexist. It would be beneficial for everyone, we wouldn't have to die, they wouldn't have to go through the effort of killing us and they could bring blood back to test it, after all the diseases in our blood probably wouldn't thrill any bloodsucking creatures regardless if they had to feed off it or not.

Edit: I should have said transform us rather than kill.

Lets say the vampires we are talking about are like the ones from van Helsing then yes it would be a problem because they take out whole villages just for blood but if it where any other vampire like from legends or other films/books it wouldn't be that bad because they would be careful not to get caught. So they would either hide the body or make it look like a animal attack and they don't happen very often so I don't think they would be much of a problem plus if they where real it is likely that others will know about them and possibly become vampire hunters and lessen the problem if there is one.

Plus who says they're not real already?

I think the best reference for this kind of debate would be either True Blood or Kim Newman's Anno Dracula books, some Vampires could be a problem the same way that there are antisocial living people. As in the books there would be people with a Vampire fetish who would willingly give their blood and drinks companies would not miss the chance to cash in so there would be some kind of Blood substitute marketed so Vampires would just become another ethnic group.

Well according to True Blood, Twilight, and The Vampire Diaries, it wouldn't be so bad, but even with those aside you have to take into consideration naturual selection and things of that sort.

If human blood is the only thing that can maintain them then they can't kill all of us. Then again, in the movie Daybreakers they ended up herding us like cattle.

Then there is the potential that as with Supernatural, the series, that they can live off of any blood. If that is the case then they will simply follow rules and not hunt humans, with the exception of criminal vampires, but we have criminal humans that hunt humans so that won't be too diffrent.

Finally I must say, if vampires existed and they were just regular people living, or not, like us then they'd be at the top of the food chain with humans as virtualy a food sorce to them, and if they chose that we had rights, then they wouldn't imprison us like in Daybreakers but instead they'd end up keeping our population from overflowing. Which I would hope people would see as a good thing.